Small Nix documentation improvements (#162)

- Make using Nix the preferred way of building kernels (over Docker).
- Add documentation for `pythonCheckInputs`.

Author: danieldk

README.md

@@ -43,8 +43,8 @@ See [dockerfiles/README.md](./dockerfiles/README.md) for more options, including
 # 📚 Documentation
 
 - [Writing Hub kernels](./docs/writing-kernels.md)
-- [Building kernels with Docker](./docs/docker.md)
 - [Building kernels with Nix](./docs/nix.md)
+- [Building kernels with Docker](./docs/docker.md) (for systems without Nix)
 - [Local kernel development](docs/local-dev.md) (IDE integration)
 - [Why Nix?](./docs/why-nix.md)
 

docs/docker.md

@@ -1,6 +1,7 @@
 # Using the kernel builder with Docker
 
 <!-- toc -->
+
 - [Using the kernel builder with Docker](#using-the-kernel-builder-with-docker)
   - [Quick Start](#quick-start)
   - [CLI Interface](#cli-interface)
@@ -16,7 +17,12 @@
   - [Building from URL](#building-from-url)
   - [Available Docker Images](#available-docker-images)
   - [Development](#development)
-<!-- tocstop -->
+  <!-- tocstop -->
+
+**Warning**: we strongly recommend [building kernels with Nix](nix.md).
+Using Nix directly makes it easier to cache all dependencies and is more
+robust. We provide a Docker image for systems where Nix cannot be
+installed.
 
 ## Quick Start
 
@@ -204,12 +210,13 @@ docker run --rm \
 
 The kernel-builder is available in different variants with specific tags:
 
-| Tag | Description |
-| --- | ----------- |
-| `[SHA]` | Specific commit hash version (example: `ghcr.io/huggingface/kernel-builder:abc123`) |
-| `user-[SHA]` | Non root user variant (use when specific permissions are needed) |
+| Tag          | Description                                                                         |
+| ------------ | ----------------------------------------------------------------------------------- |
+| `[SHA]`      | Specific commit hash version (example: `ghcr.io/huggingface/kernel-builder:abc123`) |
+| `user-[SHA]` | Non root user variant (use when specific permissions are needed)                    |
 
 All images are available from the GitHub Container Registry:
+
 ```
 ghcr.io/huggingface/kernel-builder
 ```

docs/nix.md

@@ -1,9 +1,11 @@
 # Using the kernel builder with Nix
 
 The kernel builder uses Nix for building kernels. You can build or
-run the kernels directly if you have [Nix installed](https://nixos.org/download/)
-on your system. On systems without Nix you can use the [Docker](./docker.md)
-image, which is a wrapper around Nix.
+run the kernels directly if you have Nix installed on your system.
+We recommend installing Nix in the following way:
+
+- Linux: use the [official Nix installer](https://nixos.org/download/).
+- macOS: use the [Determinate Nix installer](https://docs.determinate.systems/determinate-nix/).
 
 ## Getting started
 
@@ -35,9 +37,6 @@ cd examples/activation
 nix build . -L
 ```
 
-You can put this `flake.nix` in your own kernel's root directory to
-get add Nix support to your kernel.
-
 ## Shell for local development
 
 `kernel-builder` provides shells for developing kernels. In such a shell,
@@ -82,6 +81,39 @@ nix develop -L .#test
 python -m pytest tests
 ```
 
+## Adding test dependencies to development shells
+
+You can add test dependencies to a development or testing shell. Adapt
+the kernel's `flake.nix` to use the `pythonCheckInputs` option:
+
+```nix
+{
+  description = "Flake for my kernel";
+
+  inputs = {
+    kernel-builder.url = "github:huggingface/kernel-builder";
+  };
+
+  outputs =
+    {
+      self,
+      kernel-builder,
+    }:
+    kernel-builder.lib.genFlakeOutputs {
+      path = ./.;
+      rev = self.shortRev or self.dirtyShortRev or self.lastModifiedDate;
+
+      # The einops and numpy test dependencies are added here:
+      pythonCheckInputs = pkgs: with pkgs; [ einops numpy ];
+    };
+}
+```
+
+The available packages can be found on [search.nixos.org](https://search.nixos.org/packages?channel=25.05&query=python312Packages).
+
+Keep in mind that these additional dependencies will only be available to
+the Nix shells, not the final kernel uploaded to the Hub.
+
 ## Building a kernel without `flake.nix`
 
 If a kernels source directory does not have a `flake.nix` file, you can build the